For environment protection purposes, it is desirable to reduce a nitrogen oxide (NOx) exhausted from a gas turbine. As a method of reducing the exhaust amount of NOx, there is a method in which fuel and air are pre-mixed to generate an air-fuel mixture, and the air-fuel mixture is injected from a fuel injection nozzle and combusted (pre-mixing combustion method). In accordance with this method, by controlling the ratio of the fuel to the air-fuel mixture so that the fuel ratio becomes as low as possible, a temperature distribution in which a combustion temperature is low and uniform in the interior of a combustion chamber is obtained, and an increase in the combustion temperature can be suppressed. As a result, generation of NOx (thermal NOx) due to the increase in the combustion temperature can be suppressed (see Patent Literature 1).